This invention relates to a lubricating oil composition having improved friction reducing properties and to a method for reducing friction in internal combustion engines.
There has been considerable effort in recent years to improve the fuel economy of automotive engines which operate on petroleum fuel, a product which like other forms of energy has become very expensive. Some of the known ways to improve fuel economy have been of a mechanical or design nature, such as building smaller cars and engines. Since it is known that high engine friction causes significant energy loss, another way to improve fuel economy of automotive engines is to reduce such friction.
Major efforts to reduce friction in automotive engines have involved the lubricating oils used in such engines. One approach has been to use synthetic ester base oils which are generally expensive. Another approach has been to use additives to improve the friction properties of the lubricating oil. Among the friction reducing additives which have been used are a number of molybdenum compounds, including insoluble molybdenum sulfides and organic molybdenum complexes, e.g., molybdenum amine complexes, disclosed in U.S. Pat. No. 4,164,473; molybdenum thio-bisphenol complexes disclosed in U.S. Pat. Nos. 4,192,753, 4,201,683 and 4,248,720; molybdenum oxazoline complexes, disclosed in U.S. Pat. No. 4,176074; and molybdenum lactone oxazoline complexes, disclosed in U.S. Pat. No. 4,176,073.
Another group of friction reducing additives which have been used in lubricating oils are the carboxylic acid esters. These compounds include the esters of fatty acid dimers and glycols, as disclosed in U.S. Pat. No. 4,105,571; the esters of monocarboxylic acids and glycerol, as disclosed in U.S. Pat. No. 4,304,678; the ester of dimer acids and monohydric alcohol, disclosed in U.S. Pat. No. 4,167,486; the esters of glycerol and monocarboxylic fatty acids, as disclosed in U.K. Patent Nos. 2,038,355 and 2,038,356; and esters of monocarboxylic fatty acids and polyhydric alcohols, disclosed in U.S. Pat. No. 3,933,659. U.S. Pat. No. 4,459,223 describes friction reducing agents which are the reaction product of a dimer carboxylic acid and a polyhydric alcohol. Some friction modifiers have limited solubility in lubricating oil and, therefore, limited potential for improving fuel economy. The instant invention teaches a method for improving the solubility and stability of these friction reducing agents.
While the different approaches described above all generally provide some reduced engine friction, and consequently, improved fuel economy, there is always the need and desire for further reductions in energy losses due to friction or otherwise, since even somewhat small reductions per individual engine can result in rather significant fuel savings, particularly when considering the total number of engines in use.